1. Field of the Invention
The present invention is directed to a gradient coil system of the type having at least one shim receptacle space for passive shim elements.
2. Description of the Prior Art
Magnetic resonance technology is a known technology for, among other things, acquiring images of the inside of the body of an examination subject. In a magnetic resonance apparatus, rapidly switched gradient fields that are generated by a gradient coil system are superimposed on a static basic magnetic field that is generated by a basic field magnet. The magnetic resonance apparatus also has a radiofrequency system that emits radiofrequency signals into the examination subject for triggering magnetic resonance signals and picks up the magnetic resonance signals, on the basis of which magnetic resonance images are produced.
A high homogeneity of the basic magnetic field is a critical factor for the quality of the magnetic resonance images. Inhomogeneities of the basic magnetic field within a homogeneity volume of the magnetic resonance apparatus cause geometrical distortions of the magnetic resonance image that are proportional to the inhomogeneities. Shim systems are employed for improving the basic magnetic field homogeneity within the homogeneity volume. A distinction is made between passive and active shim systems. In a passive shim system, a number of laminae composed of a magnetic material, particularly a ferromagnetic iron alloy, are attached in the examination space of the magnetic resonance apparatus in a suitable arrangement. To that end, the basic magnetic field is measured within the homogeneity volume before the attachment of the laminae. Using the measured values, a computer program determines the suitable number and arrangement of the laminae.
During operation of the gradient coil system, the amplitudes of the required currents amount to several 100 A in the conductor of the coil. The current rise and decay rates amount to several 100 kA/s. The drive voltage for the coil current amounts to up to several kV. The gradient coil is often cooled for governing the aforementioned, high electrical powers. German OS 197 21 985 and 197 22 211 each disclose a cooling device for the indirect cooling of conductors of the gradient coil. A flexible cooling conduit following a tightly curved path through which a coolant is conducted for cooling the gradient coil is introduced on a cylindrical surface of a hollow-cylinder gradient coil system that is cast with casting resin.
German PS 100 20 264 discloses that a constant temperature of a passive shim system is important for maintaining the constant precision of the shim effect. Without counter-measures, the creation of heat in the current-carrying conductor of the gradient coil leads to a temperature variation and thus to a change in volume of the neighboring, passive shim system. The shim effect thus is no longer optimum and the homogeneity of the basic magnetic field and the quality of magnetic resonance images therefore are degraded. In order to prevent such temperature fluctuations in a passive shim system German PS 100 20 264 discloses designing the conductors of the gradient coil to be coolable andxe2x80x94at the same timexe2x80x94heat insulating the conductor from a carrier structure of the gradient coil system. A high temperature stability thus is achieved for the passive shim system arranged in the carrier structure.
An object of the present invention is to provide an improved gradient coil system that assures a constantly good shim effect for a passive shim system arranged in the gradient coil system regardless of the operating condition of the gradient coil system.
This object is achieved in accordance with the invention in a gradient coil system having at least one shim receptacle space for passive shim elements, and a decoupling device with which a change in volume of components of the gradient coil system caused by a temperature change is mechanically decoupled from the shim receptacle space and that is arranged such in the gradient coil system that the shim receptacle space remains unvaried in position relative to a defined point of a volume to be shimmed.
The invention is based on the perception that a positional change of a passive shim system arranged in the gradient coil system relative to a defined point of a volume to be shimmed as a consequence of a change in temperature and the change in volume of components of the gradient coil system accompanying the change in temperature, also leads to a degradation of the shim effect, as does the change in temperature of the passive shim system itself, as is known. Cooling of the gradient coils and/or shielding coils of the gradient coil system is incapable of completely preventing such a change in volume, since volume changes propagate across regions of cooling devices in the carrier material of the gradient coil system due to mechanical stresses, so that, for example, the entire gradient coil system lengthens. The decoupling device mechanically decouples temperature change-induced lengthening and compression tendencies of components of the gradient coil system, particularly in regions wherein conductors of gradient coils and/or shielding coils proceed, from shim receptacle spaces of the gradient coil system. A change in position of the passive shim system arranged in the shim receptacle spaces thus is prevented.